The present invention relates to apparatus for simulating solar radiation, and particularly to a solar simulator which closely approximates the characteristics of the violet and near ultraviolet portions of the solar spectrum at the earth's surface.
Solar simulators have been developed in order to provide convenient sources of radiation which can reproducibly provide the equivalent of sunlight on demand and without concern for variables such as weather conditions and sun position. Commercially-available simulators typically comprise high-pressure xenon arc lamp, a light source providing continuous radiation (with superimposed xenon emission lines) which, after filtration to reduce excessive infra-red and ultraviolet power, is used as artificial sunlight.
For applications such as the testing of organic plastics, dyes, photochromic glasses and the like, it is quite important to accurately reproduce sunlight in the ultraviolet and violet portions of the terrestrial solar spectrum, since these portions have the largest effect on the performance of such materials. Under average terrestrial sunlight conditions, taken to be sunlight at sea level with the sun 30.degree. above the horizon, solar power is largely concentrated in the visible and near infrared, and decreases rapidly in the near ultraviolet to effectively terminate at a wavelength of about 0.3 .mu.m.
This spectral termination of sunlight in the near ultraviolet is reproduced in some presently available solar simulators through the use of a filter, e.g., an interference filter, which reduces the irradiance of the simulator at wavelengths below 0.3 .mu.m to negligible values. However, correspondence between such simulators and sunlight in the wavelength region from 0.3 .mu.m to about 0.45 .mu.m is still not as good as would be desired, particularly where excess radiation is emitted in the 0.3-0.4 .mu.m range.
Even when a relatively good correspondence with sunlight is obtained, a deterioration in simulator performance may be observed over a period of time. Some of this deterioration may be attributed to a change in the performance of the ultraviolet interference filter, due to prolonged exposure of the filter to xenon arc radiation.
The addition of tin oxide to fused quartz to provide a quartz ultraviolet absorbing filter having a sharp cut-off at 2800A is described by Maddock in J. Soc. Glass Tech., 23, 372-377 (1939). However, relatively low concentrations of tin oxide were used, and only a very low wavelength portion of the spectrum, of no interest for solar simulation, was effectively modified.
It is a principal object of the present invention to provide a solar simulator which closely approximates average terrestrial sunlight, particularly in the violet and ultra-violet portions of the spectrum, but which utilizes an ultraviolet filter which is both optically stable and inexpensive to produce.
It is a further object of the invention to provide a glass absorption filter which, when used in combination with a xenon arc, provides filtered light closely approximating that of terrestrial sunlight.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.